The dangerous “knockout” attacks on strangers in large U.S. cities are leading to arrests, more officers flooding the streets and more warnings for vigilance among an unsuspecting public.

Hoodlums have dubbed the violent practice as the ‘Knockout Game,’ where teens try to randomly knock out strangers with one punch.

The attacks have raised concerns across the country. Recent attacks have occurred in New York, New Haven, Conn., Washington, D.C. and suburban Philadelphia. But the violent attacks go back several years. In 2011, St. Louis, Mo. had a rash of incidents, one of which led to the killing of a Vietnamese immigrant.

The trend may have hit the west coast with an attack Tuesday on a man in downtown San Diego, according to local news stations.